Question: If $3a + 9b = 7$ and $4x + 7y + 8z = -10$, what is $-21a + 48z - 63b + 24x + 42y$ ?
Answer: $= -21a - 63b + 24x + 42y + 48z$ $= (-7) \cdot (3a + 9b) + (6) \cdot (4x + 7y + 8z)$ $= (-7) \cdot (7) + (6) \cdot (-10)$ $= -49 - 60$ $= -109$